This invention relates to copper interconnections used in microelectronics and methods for depositing metal-containing layers.
Copper is replacing aluminum as the material of choice for wiring of microelectronic devices, such as microprocessors and memories. The copper is normally placed into holes and trenches in an insulator such as silicon dioxide, by an electroplating process. Excess copper is then polished off of the surface of the device. The structure is capped by insulation into which holes and trenches are etched to begin the next level of wiring.
In order for the tiny copper wires to survive the polishing process, the copper must adhere strongly to the insulator. Adhesion must also be maintained through the rest of the production and use of the device. In currently used technology, a bilayer structure of sputtered tantalum nitride (TaN) and tantalum metal (Ta) is used to provide this adhesion. The TaN provides strong adhesion to the insulator, and the Ta adheres strongly to a sputtered seed layer of copper, onto which further copper is electroplated. Ta also prevents oxygen and water from corroding the copper wiring.
The presence of copper in semiconductors such as silicon causes defects that can prevent the proper functioning of transistors formed in the semiconductor. Copper also increases the leakage of current through insulators, such as silicon dioxide, placed between the copper wires. Therefore use of copper wiring demands that efficient diffusion barriers surround the copper wires, to keep the copper confined to its proper locations. The sputtered TaN serves as the diffusion barrier in current technology.
Copper also has a tendency to move in the direction that electrons are flowing in a circuit. This electromigration process can lead to increased electrical resistance or even an open circuit if a sufficiently large void forms within a copper interconnection. Most of this unwanted motion takes place along the surface of the copper. Long lifetimes can be maintained by surrounding the copper interconnections by materials that inhibit electromigration. Tantalum metal (Ta) serves this function on the bottom and sides of currently-used copper interconnections. The tops of copper wiring (those parts that do not connect to an upper level) typically are covered by silicon nitride or silicon carbide, although these materials are not as effective as the Ta in reducing copper electromigration.
In future microelectronic devices, industrial planning, as published yearly in the International Technology Roadmap for Semiconductors (ITRS), calls for narrower wiring based on thinner barrier, adhesion and seed layers. The ITRS projects that currently-used sputtered Cu/TaN/Ta will not be able to meet these projected needs. The poor conformality of sputtered coatings means that thicker than necessary layers are needed near the top of holes and trenches in order to provide sufficient thickness in the lower parts of these structures. The resulting “overhang” near the tops of the features makes it difficult for electroplated copper to fill the holes and trenches without leaving voids, which increases the resistance and exacerbates the electromigration-induced instabilities.
Cobalt (Co) metal has been suggested as a replacement for the Ta adhesion layers in interconnects. Co films can be vapor-deposited (CVD or ALD) with better conformality than sputtered Ta. However when copper is vapor-deposited onto cobalt surfaces, the copper tends to agglomerate into separated nuclei forming relatively rough films with low electrical conductivity.
Ruthenium (Ru) metal has also been suggested as a replacement for the Ta adhesion layers in interconnects. Ru films can be vapor-deposited (CVD or ALD) with better conformality than sputtered Ta. When copper is vapor-deposited onto Ru, the copper layers can be smooth and highly conductive when made under appropriate conditions. However, Ru is an expensive metal, and Ru may not be available in sufficient quantities for large-scale application in interconnects. Also, Ru is not a good diffusion barrier to oxygen.
Thus current interconnect technology lacks a conformal, inexpensive adhesion and oxygen diffusion barrier layer on which smooth and highly conductive layers of copper may be deposited.